I have the following HTML structure:
$('#subgroup a').nextUntil('h3').wrapAll('<div></div>');
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<div id="subgroup">
<h3>Group name #1</h3>
<a href="#">Link #1</a>
<a href="#">Link #2</a>
<h3>Group name #2</h3>
<a href="#">Link #3</a>
<a href="#">Link #4</a>
</div>
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